The present invention relates to methods for producing a gas diffusion layer, an electrode and a membrane electrode assembly for a polymer electrolyte fuel cell, and to a gas diffusion layer, an electrode and a membrane electrode assembly produced by the aforesaid methods.
Conventional polymer electrolyte fuel cells using a cation (hydrogen ion) conductive polymer electrolyte membrane simultaneously generate electricity and heat by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air.
FIG. 17 is a schematic cross sectional view illustrating a basic structure of a unit cell designed to be mounted in a conventional polymer electrolyte fuel cell. FIG. 18 is a schematic cross sectional view illustrating a basic structure of a membrane electrode assembly designed to be mounted in the unit cell 100 shown in FIG. 17. As shown in FIG. 18, in a membrane electrode assembly 101, on each surface of a polymer electrolyte membrane 111 capable of selectively transporting hydrogen ions is formed a catalyst layer 112, which is composed of carbon powder carrying an electrode catalyst such as a platinum metal catalyst thereon and a hydrogen ion conductive polymer electrolyte.
As the polymer electrolyte membrane 111, polymer electrolyte membranes made of perfluorocarbonsulfonic acid such as Nafion (trade name) manufactured by E.I. Du Pont de Nemours & Co. Inc., USA are currently widely used. On the outer surface of the catalyst layer 112 is formed a gas diffusion layer 113 made of a conductive porous substrate such as carbon paper, carbon cloth or carbon felt, which has been previously treated for water repellency. The combination of the catalyst layer 112 and the gas diffusion layer 113 forms an electrode 114 (fuel electrode or oxidant electrode).
A conventional unit cell 100 is composed of a membrane electrode assembly 101, gaskets 115 and a pair of separator plates 116. The gaskets 115 are placed on the outer periphery of the electrodes with the polymer electrolyte membrane sandwiched therebetween so as to prevent the supplied fuel gas and the supplied oxidant gas from leaking out and to prevent them from mixing with each other. The gaskets are usually integrated in advance with the electrodes and the polymer electrolyte membrane, and the whole is sometimes called “membrane electrode assembly”.
On the outer surfaces of the membrane electrode assembly 101 are placed a pair of separator plates 116 for mechanically fixing the membrane electrode assembly 101. On the surface of the separator plate 116 in contact with the membrane electrode assembly 101 are formed gas channels 117 for supplying a reaction gas (fuel gas or oxidant gas) to the electrode and removing a gas containing an electrode reaction product and unreacted reaction gas from the reaction site to the outside of the electrodes. Although the gas channels 117 may be formed independently of the separator plate 116, they are usually formed by providing grooves on the surface of the separator plate as shown in FIG. 17.
As described above, the unit cell is formed by fixing the membrane electrode assembly 101 with the pair of separator plates 116. By supplying the fuel gas to the gas channels of one of the separator plates and the oxidant gas to those of the other of the separator plates, the unit cell can produce an electromotive force of about 0.7 to 0.8 V at a practical current density of several tens to several hundreds mA/cm2. Polymer electrolyte fuel cells, when used as power sources, are usually required to produce a voltage of several to several hundreds volts. For this reason, in practice, the necessary number of the unit cells are connected in series and clamped to give a stack for use. In the production thereof, in order to prevent gas leakage, etc, the stack of unit cells is clamped by applying a certain clamping pressure to the stack.
The gas diffusion layer 113 constituting the electrode 114 for a conventional polymer electrolyte fuel cell as described above mainly has the following three functions: (1) to diffuse a reaction gas such as a fuel gas or an oxidant gas so as to uniformly supply the reaction gas from the gas channels 117 formed outside the gas diffusion layer 113 to the catalyst in the catalyst layer 112; (2) to rapidly carry away water produced by the reaction in the catalyst layer 112 to the gas channels 117 to prevent water clogging (flooding); and (3) to transfer the electrons necessary for the reaction and the produced electrons. As such, the gas diffusion layer 113 is required to have high reaction gas permeability, high water permeability and high electron conductivity.
In order to meet such demand, in a conventional technique, gas permeability is imparted by allowing the gas diffusion layer to have a porous structure. Water permeability is imparted by dispersing a water repellent polymer as typified by fluorocarbon resin in the gas diffusion layer. Electron conductivity is imparted by using an electron conductive material such as carbon fiber, metal fiber or carbon fine powder to make the gas diffusion layer.
In view of the above, a typical gas diffusion layer is formed by coating carbon paper serving as a conductive porous substrate with fluorocarbon resin and forming a conductive water repellent layer on the catalyst-layer-side surface of the carbon paper as described in, for example, Japanese Laid-Open Patent Publication No. Hei 2-295065. The fluorocarbon resin coating is carried out to ensure water repellency for a long period of time. The conductive water repellent layer is formed to prevent an ink for forming a catalyst layer from filling and clogging the pores when a catalyst layer is formed.
In stead of carbon paper, there is a method using carbon cloth or carbon felt as the conductive porous substrate for gas diffusion layer in order to improve the properties and to achieve low cost production, as described in Japanese Laid-Open Patent Publication No. 2002-56851. In this method, a conductive porous substrate is treated for water repellency by immersing the conductive porous substrate in a water repellent agent containing a surfactant, and then drying the conductive porous substrate at a temperature at which the surfactant is not removed. A conductive water repellent layer is then formed on the aforesaid conductive porous substrate, followed by baking.
As previously mentioned, in the production of the stack, a certain clamping pressure is applied to the stack of unit cells to prevent gas leakage. When carbon paper is used as the conductive porous substrate as described above, the above-described carbon paper hardly changes in shape during the production of the stack because the carbon paper is rigid enough. Accordingly, no problem occurs during the operation of the fuel cell to be obtained.
The carbon paper, however, is excessively rigid, so the handling thereof itself is troublesome. For example, the mass productivity and the cost efficiency might be reduced when the handling of the carbon paper during the production process is difficult. Moreover, the carbon fibers constituting the carbon paper are two-dimensionally oriented, that is, oriented in the surface direction of the carbon paper. To be more specific, they are oriented in the same direction as the flowing direction of the reaction gas in the gas channels 117 of the separator plate 116. For this reason, the flow of water moving in the thickness direction of the gas diffusion layer 113, that is, the water flow from the catalyst layer 112 to the gas channels 117 of the separator plate 116, does not go smoothly, causing water to stay and collect, which makes it likely to cause flooding.
In some cases, carbon cloth or carbon felt is used as the conductive porous substrate for constituting the gas diffusion layer 113 for purposes of optimal water repellency, low cost production, process rationalization and the improvement of productivity.
Carbon cloth and carbon felt, however, have the drawback that the carbon fibers thereof are three-dimensionally oriented and therefore micro short-circuiting is likely to occur. Moreover, when the gas diffusion layer 113 is formed using carbon cloth or carbon felt, the gas diffusion layer 113 hangs down into the gas channels 117 of the separator plate 116 since carbon cloth and carbon felt are highly flexible and not rigid enough. The hanging down of the gas diffusion layer 113 causes a greater variation in pressure loss in the gas channels 117, which makes it likely to cause flooding.
There is also a problem when forming a conductive water repellent layer on the surface of the conductive porous substrate constituting the gas diffusion layer 113. The problem is that the ink for forming a conductive water repellent layer is impregnated into the conductive porous substrate, which might inhibit the gas diffusibility of the produced gas diffusion layer 113.
Moreover, it is generally considered that, in order to improve water repellency of the water repellent layer, heat treatment, i.e., baking at a temperature exceeding the melting point of the water repellent material contained in the ink for forming a water repellent layer is desirable. But when polytetrafluoroethylene (PTFE) of high molecular weight is used as the water repellent material, the problem also arises that the baking at a temperature exceeding the melting point of PTFE reduces the adhesive strength, the ease of handling and the mass productivity, and that the conductive water repellent layer is peeled and separated from the electrode, which makes it likely to cause flooding.